The principal objectives of this research project are to ascertain whether the endocrine relationships between the steroidogenic functions of fetus and placenta are comparable in sheep to those that have been established in pregnant women. Estrogens and progesterone concentrations will be determined in arterial and venous samples of uterine and umbilical blood in ewes from the 90th day of pregnancy to term. Since blood flows and plasma concentration of individual amino acids will also be determined (as part of a related project) in the resting condition and in various experimental situations, it should be possible to investigate any correlation of these parameters with the steroid hormone balance. Furthermore, the nature of possible steroid precursors in the maternal and fetal circulation, as well as the steroidogenic potential of the lamb's fetal adrenal, will be ascertained by the usual techniques (thin-layer and gas chromatographic analysis combined with spectroscopic techniques; in vitro studies with adrenal homogenates and cell-fractions; perfusion, etc.). Ethical considerations restrict the type of experimental designs which are permissible in the study of many biochemical and physiological aspects of human reproduction. Comparative studies are thus a necessity and, when carefully interpreted, are a very valuable adjunct for the understanding of reproductive phenomena in man.